This invention is based on and claims priority of Japanese patent application 2001-329688, filed on Oct. 26, 2001, the whole contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electronic device having a ferroelectric layer and a method of manufacturing the same, more particularly to an electronic device having a ferroelectric layer oriented crystallographically and a method of manufacturing the same.
2. Description of the Related Art
A semiconductor memory, in which one memory cell is constituted of one transistor and one capacitor, has been widely known. A capacitor of a dynamic random access memory (DRAM) has a capacitor dielectric layer formed of a paraelectric material. Electric charges stored in the capacitor gradually decrease therefrom due to their leak even when the transistor is turned off. Accordingly, when a voltage applied to the memory cell is removed, information stored therein decreases and disappears before long.
A memory capable of retaining information stored therein even after power is cut off is called a non-volatile memory. As a kind of the non-volatile memory, a one-transistor/one-capacitor type memory, a capacitor dielectric layer of which is formed of a ferroelectric material, has been known, which is called a ferroelectric random access memory (FeRAM).
The FeRAM utilizes residual polarization of the ferroelectric material as information stored therein. The FeRAM controls a polarity of a voltage applied between a pair of electrodes of the ferroelectric capacitor, thus controlling the direction of the residual polarization. Assuming that one polarization direction be xe2x80x9c1xe2x80x9d and the other be xe2x80x9c0xe2x80x9d, binary information can be stored. Since the residual polarization remains in the ferroelectric capacitor even after the applied voltage is removed therefrom, the non-volatile memory can be realized. In the non-volatile memory, information can be rewritten by a sufficient number of times, that is, 1010 to 1012 times. The non-volatile memory also has a rewriting speed of an order of several ten nanoseconds and offers a high-speed operability.
As ferroelectric materials, lead-based oxide ferroelectric materials having a perovskite structure and bismuth-based oxide ferroelectric materials having a bismuth-layered structure have been known. Typical examples of the lead-based ferroelectric materials are PbZrxTi1xe2x88x92xO3 (PZT), PbyLa1xe2x88x92yZrxTi1xe2x88x92xO3 (PLZT) and the like. A typical example of the bismuth-based oxide ferroelectric materials is SrBi2Ta2O9 (BST).
The ferroelectric capacitor offers a higher charge retention capability as the polarization of the ferroelectric material is greater, and can retain the electric potential with less capacitance. Specifically, the FeRAM can be fabricated with high integration. Furthermore, as the polarization of the ferroelectric material is greater, the polarization directions can be differentiated more clearly even at a low reading-out voltage, thus enabling the ferroelectric memory to be driven at a low voltage.
It is effective to arrange orientations of ferroelectric crystals uniformly in order to increase a polarization amount of the ferroelectric material. For example, on pages 382 to 388 of xe2x80x9cJournal of Applied Physicsxe2x80x9d 1991, vol. 70, No. 1, disclosed is a technology of obtaining a (111)-oriented ferroelectric thin film, in which metal thin films formed of metals such as platinum (Pt) and iridium (Ir) are deposited at 500xc2x0 C. to obtain a (111)-oriented metal thin film, and a ferroelectric thin film such as PZT is deposited on this metal thin film at a room temperature, followed by heating of the deposited ferroelectric thin film to a range from 650xc2x0 C. to 700xc2x0 C. However, the maximum temperature permitted for a manufacturing process of the FeRAM is usually 620xc2x0 C.
The ferroelectric material such as PZT having a tetragonal simple perovskite structure has a polarization axis along the c axis  less than 001 greater than . Accordingly, the polarization amount becomes maximum when the ferroelectric layer is approximately oriented along a (001) plane (hereinafter, referred to as (001)-oriented). When the ferroelectric layer is (111)-oriented, a component of the polarization produced in  less than 001 greater than  direction is only about {fraction (1/1.73)} in  less than 111 greater than  direction that is a thickness direction of the ferroelectric layer. Although the polarization can be increased by aligning orientation, it is impossible to increase the polarization to the maximum.
An object of the present invention is to provide an electronic device capable of obtaining a ferroelectric layer having a large polarization amount and a method of manufacturing the same.
Another object of the present invention is to provide an electronic device provided with a (001)-oriented ferroelectric layer and a method of manufacturing the same.
Still another object of the present invention is to provide an electronic device provided with a ferroelectric capacitor having a ReO3 layer as at least one of electrodes and a method of manufacturing the same.
According to one aspect of the present invention, there is provided an electronic device including: a ReO3 layer having a (001) orientation; and an oxide ferroelectric layer having a perovskite structure, the oxide ferroelectric layer being formed on the ReO3 layer and having a (001) orientation.
According to another aspect of the present invention, there is provided a method of manufacturing an electronic device, including the steps of: preparing a ReO3 layer having a (001) orientation; and forming an oxide ferroelectric layer having a perovskite structure on the ReO3 layer, the oxide ferroelectric layer having a (001) orientation.
A (001)-oriented MgO layer is preferably used as an underlying layer of the ReO3 layer.
Lattice matching can be made for the (001)-oriented ReO3 layer and the (001)-oriented oxide ferroelectric layer having a perovskite structure; accordingly, the (001)-oriented oxide ferroelectric layer having a perovskite structure can be formed on the (001)-oriented ReO3 layer.
The MgO layer can be easily (001)-oriented. The lattice matching can be made for the (001)-oriented MgO layer and the (001)-oriented ReO3 layer. Hence, the (001)-oriented ReO3 layer and the (001)-oriented oxide ferroelectric layer having a perovskite structure can be formed on the (001)-oriented MgO layer sequentially.
The term xe2x80x9cReO3xe2x80x9d used herein includes ReO3 to which metal other than Re is added, for example, for controlling a lattice constant thereof.
In such a manner as described above, it is possible to form a ferroelectric capacitor capable of realizing greater polarization.